The present invention relates to the construction of a print head for use in a dot matrix printer or the like which is modular in nature, permitting variable length printers to be assembled by stacking standard components to a desired degree.
A dot matrix printer is an apparatus which prints a plurality of closely spaced dots at high speed at selected locations on a paper strip to form letters, numerals or other intelligible symbols thereon. The dots are formed by causing contact between the paper and an ink impregnated surface at the desired locations by selectively electromagnetically displacing elongated print wires mounted within the print head.
Certain types of known dot matrix print heads consist of plurality of selectively electrically energizable solenoids, each of which has a separate print wire extending therefrom. The impact ends of the print wires are retained in a fixed position with respect to each other by a stationary wire bearing which forms a part of the head. The wire bearing has a plurality of closely spaced openings, arranged in a matrix array. Each opening receives the impact end of a different print wire. Energization of a selected solenoid results in the print wire associated therewith being displaced, such that the impact end thereof extends beyond the surface of the bearing and causes contact between the paper and the ink impregnated surface. The paper is moved relative to the wire bearing in a first direction, such that printing of symbols can take place along a line and in a second direction such that different lines can be printed.
The type of hammer comprising a solenoid and print wire, as shown in U.S. Pat. No. 4,349,283 of Sept. 14, 1982, entitled "Hammer for Dot Matrix Printer", assigned to the assignee of this application, may be considered exemplary of the type of hammer means suitable for use in connection with the present invention, but the present invention is not limited thereto. In addition, in copending application Ser. No. 485,749 of Apr. 18, 1983, abandoned and refiled as Ser. No. 690,416, which issued as U.S. Pat. No. 4,534,287 on Aug. 13, 1985, entitled "Improved Dot Matrix Print Head" and applied for in the name of Robert A. Meloni, also assigned to the assignee of this application, a particular type of head is disclosed in which the ends of the print wires are secured within a low inertia member which, when moved, positions the impact ends of the print wires, and the device of the present invention will be here specifically disclosed in connection with such an improved print head, but here again the present invention is not limited to use with a print head of that type.
Hammers of the type particularly applicable to the present invention are extremely thin, coil carrying hammers.
One problem which has arisen in making heads including hammers of the type described arises from the fact that a plurality of those hammers are interpositioned between a single pair of stationary magnets, the spacing between the magnets being dependent upon the number of hammers interpositioned therebetween and that spacing thus increasing as the number of hammers is increased. As the magnet spacing increases, the strength of the magnetic field active on the hammers, and particularly the hammers at the middle of the stack, decreases. This in turn increases the requirement for the minimum signal strength to be applied to each individual hammer solenoid, and also tends to lessen the speed with which the hammer responds when a signal is applied thereto. This places a limitation on the number of hammers that can effectively be interpositioned between a given pair of magnets.
Moreover, each time that a unit is designed with a different number of hammers, the entire print head structure must be redesigned in order to accommodate the new hammer and spacing requirements. Tooling and design costs become significant.
It is a prime object of the present invention to provide a dot matrix print head of modular construction, one which may be built up to utilize any desired number of hammers through the use of standard components and without requiring redesign or modification of the parts thereof.
It is a further object of the present invention to provide a print head construction which can be easily modified to provide different print formats, by increasing or decreasing the number of characters or to increase or decrease print speed by changing the number of characters printed, without requiring redesign of the head parts.
It is yet another object of the present invention to provide a print head formed of a plurality of sturdy structural elements which may be easily and inexpensively manufactured, and to no excessive degree of precision.
It is a further object of the present invention to provide a dot matrix print head formed of structural frame elements stackable one upon the other to any desired degree, those elements being so constructed that when they are stacked, spaces are provided between them within which solenoid-type hammers may be received and from which operative portions of those hammers may freely movably extend, the elements themselves carrying the magnets which cooperate with the solenoids.
To these ends the print head is formed of a plurality of sturdy non-magnetic frame pieces each carrying magnets which, when the frame pieces are stacked one on the other, cooperate with one another to define the magnetic field within which the solenoid-type hammers function. The stack of hammers is so constructed that there are spaces formed between the magnets of adjacent frames, within which spaces flat solenoid-type hammers are freely movable received, and the periphery of the frames, when stacked, define openings through which electrical access to the solenoids may be achieved and through which the print wires and/or their supporting structure may freely movably extend to points outside the head where the impact ends of the print wires may perform their printing function. Any desired number of frames and magnets can be formed into a stack, depending upon the number of hammers desired in a given piese of apparatus. Because the frames carry magnets, the magnetic field active on each solenoid is quite strong even though a very large number of hammers may be employed in a given place of apparatus, and hence optimum hammer speed and optimum minimal signal magnitude are achieved. Because the head is built up to desired length from a plurality of similar items, with the same items being used whether the number of hammers is great or small, significant manufacturing economies are achieved.